Rails 4 or Rails 3?

Today, I've been working with http://guides.rubyonrails.org/index.html. The last version of the guide uses Rails 4 and Volontari uses Rails 3.

First, I thought that I was able to make the blog of the guide with Rails 3, until I arrived to Strong Parameters. I had an error with these parameters, so I decided to use Rails 4.
For updating Rails, I used http://railscasts.com/episodes/415-upgrading-to-rails-4

I didn't know if updating Rails was going to affect to Volontari app. I solved the problem with the blog of the guide, but Volontari, didn't work. So I made a branch in Github, I installed with bundle, started the rails server and it worked. I don't want to make a commit to Volontari until I speak with Mathias.

Then, I continued until point 5.12. of the guide.

Although I was able to continue when I had an error, I have a lot of doubts about how Rails works. So, I'll have to ask Mathias about that. I think that today has been a good day of work and I was able to see how easy is making an application with Rails.